1. Field of the Invention
This invention generally relates to radio frequency (RF) antennas and, more particularly, to high frequency (HF) antennas for wideband signals used in the tracking of railcars.
2. Description of the Prior Art
U.S. Pat. No. 5,588,005 to Ali et al. and assigned to the instant assignee describes the tracking of assets, including goods and vehicles, using the Global Positioning System (GPS). While goods are an example of assets that need to be tracked, the containers, container trucks and railcars in which the goods are shipped are themselves assets which need to be tracked, not just because of the goods they carry, but also because they represent capital assets typically of a leasing company not associated with the carrier.
The mobile tracking unit used in the Ali et al. system includes a navigation set, such as a Global Positioning System (GPS) receiver or other suitable navigation set, responsive to navigation signals transmitted by a set of navigation stations which can be either space-based or earth-based. The navigation set is capable of providing data indicative of the vehicle location, based on the received navigation signals. In addition, the mobile tracking unit can include a suitable electromagnetic emitter for transmitting to a remote location the vehicle position data and other data acquired through use of sensing elements in the vehicle.
One of the options available for tracking railcars involves transmission of a wideband HF signal from the tracked railcar. The frequency of interest and the railcar environment make the problem of efficient antenna design especially difficult. Neither a whip antenna or large erect loop antenna is deemed a viable candidate as it would be expected to suffer environmental damage and/or exceed the template limits for the railcar which are set by the railcar industry in order that the railcar may traverse rail systems of appropriate gauge without ordinarily suffering or causing damage. As the antenna is a critical element of such communications system, what is needed is a suitable antenna which is especially adapted for use on a railcar.
A primary requirement for the antenna is that it be rugged. The railcar environment is notoriously hostile. Any antenna mounted on a railcar would be subject to vibration, shock, abrasion, high humidity, extreme dryness, heat, cold and vandalism.